Effects of feeding dry-rolled corn-based diets with and without wet distillers grains with solubles and zilpaterol hydrochloride on performance, carcass characteristics, and heat stress in finishing beef steers1

Zilpaterol hydrochloride (ZH) has been approved for use since 2006; however, there is no research on any interactions between ZH and co-products such as wet distillers grains and solubles (WDGS). Additionally, there is no published information on the potential effects of ZH on heat stress in feedlot cattle. Therefore, an experiment was conducted to determine the effects of feeding dry-rolled corn-based diets with and without WDGS and ZH on performance, carcass characteristics, and heat stress in finishing beef steers. Four hundred thirty-eight steers were used (415.3 ± 2.1 kg BW) in a randomized complete block design with a 2 × 2 factorial arrangement of treatments. Factors consisted of inclusion of WDGS 0% or 30% on a DM basis and inclusion of ZH at 0 or 84 mg/steer daily for 21 d at the end of the finishing period. Thus, cattle were blocked by BW and randomly assigned to 1 of the resulting 4 treatment combinations: 1) dry-rolled corn (DRC)-based diet with 0% WDGS and 84 mg/steer ZH (CON+ZH), 2) DRC-based diet with 0% WDGS and no ZH (CON-ZH), 3) DRC-based diet with 30% WDGS and 84 mg/steer of ZH (WDGS+ZH), and 4) DRC-based diet with 30% WDGS and no ZH (WDGS-ZH). Final live BW, carcass-adjusted BW, ADG, and G:F were greater for cattle fed ZH than non-ZH fed cattle (P < 0.01). Additionally, cattle fed ZH consumed 7.4% less DM than cattle not fed the additive (P < 0.01). Cattle fed ZH for 21 d also had a 2.9% greater HCW (P < 0.01), a 1.1% greater dressing percentage (P < 0.01), 7.3% greater LM area (P < 0.01), and an 8.4% improvement in yield grade (P < 0.01) than cattle not fed ZH. For the main-effect of WDGS inclusion, ADG was greater for cattle fed 0% vs. 30% WDGS (P = 0.04) and G:F also tended to be greater for cattle fed 0% vs. 30% WDGS (P = 0.07) for the 21 d ZH feeding period. However, when evaluated over the entire experiment, cattle fed 30% vs. 0% WDGS had a greater ADG and G:F (P < 0.01). Furthermore, cattle fed 30% vs. 0% WDGS had a greater dressing percentage and tended to have a greater amount of 12th rib fat (P < 0.07). The slopes for change in respiration rate and panting score per day were positive, but not different across dietary treatments (P = 0.57 and P = 0.54, respectively). Likewise, the slopes for change in respiration rate and panting score when accounting for environmental conditions were positive, but not different across dietary treatments (P = 0.46 and P = 0.32).